Magnetic tape cartridges provide a means to store data on magnetic tape to be saved and read back at a subsequent time. A magnetic tape drive writes the data to magnetic tape, typically as a set of parallel tracks, and subsequently a magnetic tape drive reads back the data. To read back the data, a magnetic tape drive typically comprises parallel read heads to read each of the parallel tracks, a drive system for moving a magnetic tape with respect to the read heads such that the read heads may detect magnetic signals on the magnetic tape, and a read channel for digitally sampling magnetic signals detected by the read heads and for providing digital samples of the magnetic signals. The digital samples are then decoded into data bits, and the data bits from the parallel tracks are combined to provide the read back data. The read channel typically requires an equalizer for each of the read heads to compensate for the change in the signal due to the magnetic recording properties of the write head, the magnetic tape, and the read head. Magnetic tapes may be interchanged between tape drives, such that a magnetic tape written on one tape drive will be read by another tape drive. Variation in the response of the read heads to the variously written magnetic tapes may result in unacceptably poor read back of the recorded signals.
Adaptive equalizers have been implemented in magnetic tape drives, and have been based on solving a set of equations to find the equalizer characteristic that reduces the error between the desired and actual amplitudes. The set of equations may be highly complex and require some amount of time to calculate. Thus, the equalizer might be computed at the beginning of use with respect to a magnetic tape, or recomputed a few times during use. Further, the desired amplitudes may be difficult to estimate. Hence, in many instances, the desired amplitudes are best estimated by employing a signal having known characteristics, such as a synchronization signal, or a data set separator signal, and not the random data signals.
In magnetic tape, the recording characteristics may not only vary from track to track, but may as well vary in a continuous fashion along a track or tracks. Thus, a selected equalizer characteristic, although satisfactory at the beginning or at some specific track location of a magnetic tape, may lead to an increase in data read errors at some point along the track.
Further, in magnetic tape, an equalizer typically equalizes signals in the asynchronous domain, which means that the digital samples that are processed by the equalizer are taken asynchronously with respect to the clock that is used to write the data on the magnetic tape. This makes a determination of a desired amplitude at the point of the asynchronous sample a difficult task.